docs: epoll's shutdown now sends what the sockets have not taken yet, until the deadline (celeris#760)

[FumingPower3925]

Matches goceleris/celeris#807 (fixes celeris#760). Merge after it.

epoll's shutdown (and adaptive's while it runs epoll) now sends what the sockets have not taken yet before it closes the connections, while the shutdown's context is live: until its deadline, or, for a ctx with no deadline such as context.Background(), until that ctx is done; never for longer than Config.WriteTimeout nor for less than 250 ms. The graceful-shutdown page described the old gap ("each connection is then closed without flushing what the socket has not taken yet"); it now says what the native engines do, keeps the old behaviour as the pre-v1.6.0 note, and names io_uring's own 250 ms bound (celeris#806). The shutdown sequence's step 3 and the FAQ's deadline answer gain the send step. The measured line gains the new result (the same 3 MiB response to a 64 KiB-receive-buffer client that starts reading 1 s later arrives whole on std, epoll and adaptive; on io_uring it depends on the kernel, and a CI runner cut it at 2,634,119 of 3,145,728 body bytes: celeris#806's comment).

Round 2 (celeris#807's review): the no-deadline case (a Shutdown(context.Background()) got only the 250 ms floor before #807's round 2) and the WriteTimeout bound are added to the paragraph.

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Clarified when buffered responses are sent during graceful shutdown and how flush limits differ between epoll, adaptive and io_uring.
  • Updated shutdown measurements and FAQ guidance to explain that, after a deadline expires, epoll and io_uring connections get up to 250 ms to send responses before closing. Responses larger than the socket buffers may be cut off.

Walkthrough

The graceful shutdown guide describes when native engines send buffered response data, gives engine-specific flush limits, and notes that slow clients may receive truncated responses after a deadline expires.

Changes

Graceful shutdown documentation

Layer / File(s)	Summary
Shutdown sequence and response flushing src/content/docs/graceful-shutdown.md	Lines 221–223 describe sending buffered response data after handlers finish and before connections close. Lines 269–293 document flush limits and updated measurements for epoll, adaptive, and io_uring. Lines 672–675 describe the 250 ms allowance after a deadline expires and possible response-tail loss for slow clients.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Merge Risk: 🔵 Low · up to d897b

The shutdown guide may mislead users about timing guarantees and response delivery in some edge cases. Examples include io_uring clients that stop reading, a disabled WriteTimeout, and HTTP/2 streams on async routes. Correct these statements before relying on the guide, but the change carries no runtime risk.

Architecture Summary

Architecture risk: 🔵 Low · up to d897b

The change affects 1 system.

Changed systems: src

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — src (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in src/content/docs/graceful-shutdown.md: The native-engine shutdown description now says handlers are followed by sending data not yet accepted by the sockets before connections close; previously it said connections closed before async handlers were awaited.
• observed — Modified behavior in src/content/docs/graceful-shutdown.md: The drain documentation replaces the claim that epoll and adaptive immediately close connections without flushing with engine-specific response flushing: epoll and adaptive send while the shutdown context is live, subject to Config.WriteTimeout and a 250 ms minimum, while io_uring sends for 250 ms regardless of the deadline. The measured results are revised to show complete responses on epoll and adaptive in the stated test, and variable results on io_uring.
• observed — Modified behavior in src/content/docs/graceful-shutdown.md: The expired-deadline FAQ now specifies that epoll and io_uring allow 250 ms for the response to send before closing the connection, and that a response larger than socket buffers may lose its tail when the client reads slowly.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title uses the valid docs: prefix and accurately describes the change in src/content/docs/graceful-shutdown.md, but its summary is descriptive rather than imperative.	Rewrite the summary as an imperative action, for example: docs: document epoll shutdown flushing until the deadline (celeris#760). Line information was not provided.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description directly explains the shutdown documentation changes in src/content/docs/graceful-shutdown.md, including engine-specific limits, measurements, and FAQ updates.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Benchmark Provenance	✅ Passed	PASS. The pull request changes only src/content/docs/graceful-shutdown.md. Lines 269–292 and 669–673 add shutdown timing and response-byte measurements, not requests-per-second, latency percentiles,…

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Actionable comments posted: 4

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Inline comments:
Review comments at @src/content/docs/graceful-shutdown.md:
- Line 275: Update the graceful-shutdown documentation’s `WriteTimeout`
statement to qualify the cap: it applies only when `WriteTimeout` is enabled and
exceeds 250 ms. Note that disabled timeouts or shorter configured values do not
impose that cap, and preserve the documented 250 ms minimum.
- Around line 280-281: Update the io_uring drain description in the
graceful-shutdown documentation to present 250 ms as the intended drain budget,
not a guaranteed close bound, and state that a worker stalled in SubmitAndWait
on a SEND to a non-reading client can exceed it until Celeris issue #806 is
fixed.
- Around line 221-223: Clarify the `Shutdown` description and the FAQ’s
still-running-handler explanation: the drain waits for async dispatch handlers,
but does not wait for HTTP/2 streams on async routes, whose responses may be
lost by native engines. Preserve this exception in both places.
- Around line 289-293: Update the measurement paragraph in the graceful-shutdown
documentation to support the 1-second and io_uring results with a separate test
or log that identifies the kernel and architecture, or correct the figures to
match available evidence. Do not attribute the pre-fix epoll/adaptive
negative-control result to io_uring.

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📒 Files selected for processing (1)

• src/content/docs/graceful-shutdown.md

🔗 Linked repositories identified

CodeRabbit considers these linked repositories for cross-repo context during reviews:

• goceleris/celeris (manual)
• goceleris/probatorium (manual)
• goceleris/loadgen (manual)

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🧰 Additional context used

📓 Path-based instructions (2)

These pages document the Celeris framework (goceleris/celeris, linked below).

⚙️ CodeRabbit configuration file

Files:

• src/content/docs/graceful-shutdown.md

Source excerpt: Benchmark provenance: Fail if the pull request adds or changes a benchmark figure (requests per second, a latency percentile, memory or CPU usage, or a ratio or percentage comparing Celeris with another framework) in src/con...

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Files:

• src/content/docs/graceful-shutdown.md

🪛 LanguageTool

src/content/docs/graceful-shutdown.md

[uncategorized] ~270-~270: Use a comma before ‘yet’ if it connects two independent clauses (unless they are closely connected and short).
Context: ... sending what the sockets have not taken yet before they close the connections, so a...

(COMMA_COMPOUND_SENTENCE)

---

[misspelling] ~284-~284: Use “A” instead of ‘An’ if the following word doesn’t start with a vowel sound, e.g. ‘a sentence’, ‘a university’.
Context: ... one request in flight at the shutdown. An h2c request on an .Async() route got ...

(EN_A_VS_AN)

🔀 Multi-repo context goceleris/celeris, goceleris/probatorium, goceleris/loadgen

Linked repositories findings

goceleris/celeris

• server.go:512-537 documents the same contract: native engines flush after handlers return; epoll/adaptive are bounded by the shutdown context and Config.WriteTimeout with a 250 ms minimum, while io_uring uses a 250 ms bound. [::goceleris/celeris::]
• shutdown_send_drain_linux_test.go:43 tests complete 3 MiB delivery for std, epoll, and adaptive; TestShutdownSendDrainIsBounded verifies stalled clients do not exceed the shutdown budget. [::goceleris/celeris::]
• resource/config.go:62-64,302 confirms the default WriteTimeout is 60 seconds, supporting the documented distinction between context-bound and write-timeout-bound draining. [::goceleris/celeris::]

goceleris/probatorium

• The Celeris benchmark adapter overrides framework defaults to WriteTimeout: 30s and ShutdownTimeout: 10s in servers/celeris/server.go:113-121; its signal handler calls Shutdown with a 10-second context at lines 141-152. Any benchmark-specific documentation should use these values rather than framework defaults. [::goceleris/probatorium::]
• Benchmark teardown force-kills remaining server processes after a short TERM grace period (ansible/tasks/run_bench_cell.yml:150-193), so teardown is not a reliable way to measure the full documented drain window. [::goceleris/probatorium::]

goceleris/loadgen

• Its HTTP/1.1 client expects the declared Content-Length and reads the complete body (h1client.go:398-410); truncated shutdown responses therefore surface as failed/incomplete responses rather than successful partial measurements. [::goceleris/loadgen::]